Tin-coupled polymers are known to provide desirable properties, such as improved treadwear and reduced rolling resistance, when used in tire tread rubbers. Such tin-coupled rubbery polymers are typically made by coupling the rubbery polymer with a tin coupling agent at or near the end of the polymerization used in synthesizing the rubbery polymer. In the coupling process, live polymer chain ends react with the tin coupling agent thereby coupling the polymer. For instance, up to four live chain ends can react with tin tetrahalides, such as tin tetrachloride, thereby coupling the polymer chains together.
The coupling efficiency of the tin coupling agent is dependant on many factors, such as the quantity of live chain ends available for coupling and the quantity and type of polar modifier, if any, employed in the polymerization. For instance, tin coupling agents are generally not as effective in the presence of polar modifiers. The amount of coupling which is attained is also, of course, highly dependent upon the quantity of tin coupling agent employed.
Each tin tetrahalide molecule is capable of reacting with up to four live polymer chain ends. However, since perfect stoichiometry is difficult to attain, some of the tin halide molecules often react with less than four live polymer chain ends. For instance, if more than a stoichiometric amount of the tin halide coupling agent is employed, then there will be an insufficient quantity of live polymer chain ends to totally react with the tin halide molecules on a four to one basis. On the other hand, if less than a stoichiometric amount of the tin halide coupling agent is added, then there will be an excess of live polymer chain ends and some of the live chain ends will not be coupled.
Conventional tin coupling results in the formation of a coupled polymer which is essentially symmetrical. In other words, all of the polymer arms on the coupled polymer are of essentially the same chain length. All of the polymer arms in such conventional tin-coupled polymers are accordingly of essentially the same molecular weight. This results in such conventional tin-coupled polymers having a low polydispersity. For instance, conventional tin-coupled polymers normally having a ratio of weight average molecular weight to number average molecular weight which is within the range of about 1.01 to about 1.1